1. Field of the Invention
The present invention relates to a mask for EUV (Extreme Ultra Violet) exposure and a method for repairing a mask for EUV exposure. Specifically, the present invention relates to a method for repairing defects developed in a multi-layer film of a mask for EUV exposure.
2. Description of the Related Art
In order to further reduce feature sizes for semiconductor devices, other lithography methods have been recently suggested, such as direct writing with an electron beam writer (EB), cell projection, and soft X-ray reduction exposure (Extreme Ultra Violet Lithography: EUVL).
In particular, EUVL, which is a photolithography technology that utilizes a reflective optical system with soft X-rays having a wavelength of approximately 13-14 nanometer (nm), has attracted attention since it is considered to be the utmost short-wavelength limit in performing exposure with ultraviolet light.
A mask for use in EUVL has: a Mo/Si multi-layer film which is formed by alternately depositing a molybdenum and a silicon on a quartz substrate; an absorber such as TaN and TiN; and a buffer layer formed from Ruthenium (Ru), silicon dioxide (SiO2), chrome (Cr), or the like between the Mo/Si multi-layer film and the absorber, in which patterns are formed on the absorber to form a reflective mask. The thickness and film-forming condition of the Mo/Si multi-layer film are optimized based on the illuminating conditions (i.e. wavelength) and the stress balance due to thermal load.
When defects are present on the surface of the EUVL mask substrate or within the Mo/Si multi-layer, even if the defects would be a negligible for conventional photomasks, the defects may adversely affect the optical path and thus be fatal.
In particular, the defects in the Mo/Si multi-layer film include amplitude defects which cause the reduction in intensity of reflected EUV light due to foreign matter contamination in the vicinity of the surface of the multi-layer film and phase defects which are caused by propagation of a concave or convex shape in the multi-layer film surface associated with the change of the inner structure of the multi-layer film due to foreign matter on the substrate, foreign matter contamination in an early stage of the formation of the Mo/Si multi-layer film or surface defects known as pits or divots in the substrate surface.
Since the amplitude defects and phase defects may cause wafer yield degradation during wafer printing, they need to be removed before the formation of the absorber and buffer layer by performing defect repair.
With regard to such defect repair, Japanese Patent Application, First Publication No. 2006-059835 (hereinafter, referred to as Patent Document 1) describes that an electron beam or the like is locally irradiated onto a foreign matter present in the multi-layer film to form silicide at the irradiation portion for eliminating difference in level caused due to volume contraction.
In addition, Japanese Patent Application, First Publication No. 2006-060059 (hereinafter, referred to as Patent Document 2) describes that laser light having a wavelength at which the light is not absorbed by a glass substrate is converged and irradiated onto a foreign matter present on the glass substrate from the rear surface thereof and then it is heated to form silicide at the interface of the Mo/Si multi-layer film for mitigating difference in level on the Mo/Si multi-layer film.
In EUVL, the reflection efficiency of the multi-layer film is limited by the structural factors and the optical properties of the film layers. For this reason, there is a demand to increase the reflectivity of the multi-layer film. In this regard, particles located between the layers of the multi-layer film or under the multi-layer film or surface defects in the substrate surface, impact the topography of each deposited multi-layer, or variations of the optical properties of the materials may cause defects in the multi-layer film structure and/or non-uniformity of the film thickness, which result in a reduction in reflected light intensity.
The intensity of reflected EUV energy is significantly affected by the above-described factors, and changes in reflectivity may appear as an under exposed image on a wafer during the photolithography exposure processes. The technologies described in Patent Documents 1 and 2 are not sufficient to perform such a local repair of multi-layer defects.
The present invention was made in view of the above-described circumstances and the object thereof is providing a method for locally improving EUV exposure intensity in the defective region.
Another object of the present invention is providing a mask for EUV exposure capable of suppressing the occurrence and severity of waver level defects.